The present invention relates to a method for modifying surface properties in shaped articles made of a polyvinyl chloride resin or, more particularly, to a method in which the surface of a shaped article of a polyvinyl chloride-based resin is subjected to chemical modification so as that migration and bleeding of a plasticizer or other additive ingredients formulated in the shaped article on to the surface of the shaped article in the lapse of time is prevented effectively or the plasticizer or other additive ingredients thus exuded on the surface of the article are prevented from transfer to the surface of the other body being used in contact with the shaped article of polyvinyl chloride resin.
Needless, to say, vinyl chloride-based resins belong to one of the most inexpensive classes of synthetic resins so that they are used extensively in a large quantity as a resin material for fabricating a variety of shaped articles such as films, sheets, plates and tubes as well as shaped articles of complicated or irregular forms to be used in very much diversified fields of applications.
Vinyl chloride-based resins are, however, rarely fabricated into shaped articles as such but it is a very common practice that a vinyl chloride-based resin is fabricated as formulated with several kinds of additive ingredients. In particular, flexible type shaped articles of vinyl chloride-based resins are fabricated with admixture of a relatively large amount of a plasticizer ranging usually from 15 to 50% by weight of the vinyl chloride-based resin.
One of the unavoidable problems in such a high loading of a plasticizer in shaped articles of flexible type fabricated with a vinyl chloride-based resin is the bleeding of the plasticizer on to the surface of the shaped article in the lapse of time. The plasticizer thus exuded on the surface of a shaped article is naturally transferred to the surface of a second body kept in contact with the plasticizer-formulated shaped article of a vinyl chloride-based resin.
For example, many of electric wires are provided with an insulating layer of a plasticized vinyl chloride-based resin. The plasticizer in the insulating layer migrates toward the surface and dissipates into the atmospheric air, especially, accelerated by heat when the electric wire is used with a relatively large electric current. Such a loss of the plasticizer is of course undesirable for maintaining the good physical properties of the insulating layer. Therefore, the use of a less volatile high polymeric plasticizer is recommended particularly in the formulation for the insulating layers of electric wires.
Further, films and sheets of a plasticized vinyl chloride-based resin are sometimes subject to a serious problem of surface stain by the plasticizer exuded on the surface when, for example, the film or sheet is used as a flooring material or wall material. In addition, a phenomenon of so-called blocking is unavoidable with films or sheets of a plasticized vinyl chloride-based resin when the films or sheets are kept as stacked on each other or as a roll due to the stickiness by the plasticizer exuded on the surface.
Moreover, when a shaped article of a plasticized vinyl chloride-based resin is lastingly kept in contact with certain liquids such as oils and organic solvents, the plasticizer contained in the shaped article is extracted into the liquid in contact therewith so that the content of the plasticizer in the shaped article is decreased leading to lowered physical properties of the shaped article. Such an extraction of the plasticizer into the liquid is disadvantageous, especially, when the shaped articles of the plasticized vinyl chloride-based resin are containers of cosmetic materials, wrapping materials for foodstuff, medical instruments and the like because the extracted plasticizer contaminates a solution containing oil-solubilizing ingredients such as soap solutions, foodstuffs, such as milk and edible oils and physiological liquids such as blood and other body fluids.
Besides, films of plasticized vinyl chloride-based resins are widely used in the fields of agriculture such as in the building of greenhouses but the bleeding of the plasticizer is also undesirable in this case because bleeding of the plasticizer causes blocking of the film or decrease in the transparency of the films to light.
The above explanation is given particularly with respect to the plasticizer bleeding but the other kinds of additives used in the formulation of vinyl chloride-based resins are more or less subject to a similar problem of surface bleeding or blooming.
Various attempts have been made hitherto to solve the above described problems of surface bleeding or blooming of additive ingredients, typically a plasticizer, formulated in shaped articles of a vinyl chloride-based resin.
For example, the use of a high polymeric plasticizer such as polyester-based ones is recommended instead of conventional low molecular weight plasticizers in consideration of the extremely small migration and extractability. Unfortunately, such a high polymeric plasticizer is inferior in its plasticizing effect in addition to the expensiveness in comparison with low molecular weight plasticizers.
Alternatively, a method has been proposed in which the surface of shaped articles of a vinyl chloride-based resin is coated with the other kinds of synthetic resins such as acrylic resins, polyurethane resins, polyamide resins and the like though with disadvantages that such an overcoating is provided only with a burdensome operation in addition to the rather weak mechanical properties of the overcoating layer to become peeled off in a long run use or by contacting with other hard bodies.
Further alternatively, formulation of a considerable amount of certain bleeding-preventing additives is recommended such as an aliphatic acid amide, silicone fluid, silica powder, diatomaceous earth, kaolin, talc and the like in the vinyl chloride-based resin. The effectiveness of this method is not so high as desired and sufficient effects are obtained only with sacrifice of the other advantageous properties possessed inherently by vinyl chloride-based resins.
Recently there have been proposed methods in which the surface of a shaped article of a vinyl chloride-based resin is irradiated with actinic rays such as ionizing radiations, electron beams and ultraviolet light. These methods, however, have their respective problems. For example, ionizing radiations and electron beams cause crosslinking reaction of the polyvinyl chloride molecules not only in the surface layer of the article but also in the depth of the body due to their excessively high energy so that the desirable flexibility of the shaped articles of a plasticized polyvinyl chloride resin is largely lost. Furthermore, the irradiation with ultraviolet light is not free from the problem of coloring of the shaped article as a result of the degradation of the polymer molecules on the surface.
Some of the inventors have recently proposed a method in which the surface of a shaped article of a plasticized polyvinyl chloride resin is exposed to low temperature plasma of a gas so that the bleeding of the plasticizer on the surface of the article is greatly reduced (see Japanese Patent Disclosure No. 55-16004 or U.S. Patent Application Ser. No. 51,150, now allowed). This method of plasma treatment is very advantageous over the other prior art methods in that a highly crosslinked layer is formed on the surface of the treated article without coloration and not affecting the properties of the shaped article such as mechanical strengths.
Despite the outstanding effectiveness of the method of the plasma treatment, however, the method has still a problem that the effect for preventing bleeding or exudation of the plasticizer obtained by the method has rather poor durability. That is, when a plasma-treated article provided with a sufficiently highly crosslinked surface layer is used in an outdoor environment or irradiated with ultraviolet light for a long period of time, the crosslinked layer on the surface is unavoidably subject to degradation leading to the disappearance of the performance for preventing bleeding of the plasticizer. Accordingly, the practicability of the method of the plasma treatment is relatively low when high anti-weathering resistance or anti-ultraviolet resistance is required for the shaped articles such as electric wires for outdoor use, building materials for agricultural greenhouses and the like.